Coating solution for metal surfaces

Abstract

An acidic aqueous coating solution for forming a coating on an aluminum surface which is corrosion resistant and to which overlying coatings adhere excellently. The coating solution contains a zirconium and/or hafnium compound, a fluoride compound, and a polyhydroxy compound having no more than 7 carbon atoms. The coating solution is capable of forming on an aluminum surface a uniformly colorless and clear coating so that the coated surface has the appearance of the underlying metal surface, that is, the coating can be formed without changing the appearance of the metal surface. When coating a bright shiny aluminum surface, there can be produced a coated surface having a uniformly bright shiny appearance which is maintained even after the coated surface is subjected to boiling water. Such surface is capable of undergoing the "muffle test" to confirm the presence of the clear and colorless coating.

What is claimed is: 
     
       1. An aluminum surface having a uniformly colorless and clear coating which is free of boron and phosphate and is capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for 5 minutes, but which resists blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F., said coated aluminum surface having been obtained by contacting such aluminum surface with an acidic aqueous coating solution having a pH within the range of about 3.0 to about 5.0 and consisting essentially of at least about 0.5×10 -3  moles/liter of zirconium or hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of a polyhydroxy compound having no more than 7 carbon atoms, and at least sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution, wherein said polyhydroxy compound is selected from the group consisting of gluconic acid, a salt of gluconic acid, sorbitol, mannitol, dextrose, ethylene glycol, glycerine, and glucoheptonate. 
     
     
       2. An aluminum surface as in claim 1 wherein zirconium is present in the coating solution employed to coat the surface. 
     
     
       3. An aluminum surface as in claim 2 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       4. An aluminum surface as in claim 1, 2 or 3 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       5. An aluminum surface as in claim 2 wherein the source of zirconium is ammonium fluozirconate. 
     
     
       6. An aluminum surface as in claim 5 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       7. An aluminum surface as in claim 5 or 6 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       8. An aluminum surface as in claim 2 wherein the source of zirconium is fluozirconic acid. 
     
     
       9. An aluminum surface as in claim 8 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       10. An aluminum surface as in claim 8 or 9 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       11. An aluminum surface as in claim 2 wherein the polyhydroxy compound is selected from the group consisting of gluconic acid and salts of gluconic acid. 
     
     
       12. An aluminum surface as in claim 11 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       13. An aluminum surface as in claim 11 or 12 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       14. An aluminum surface as in claim 5 wherein the polyhydroxy compound is selected from the group consisting of gluconic acid and salts of gluconic acid. 
     
     
       15. An aluminum surface as in claim 14 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       16. An aluminum surface as in claim 14 or 15 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       17. An aluminum surface as in claim 8 wherein the polyhydroxy compound is selected from the group consisting of gluconic acid and salts of gluconic acid. 
     
     
       18. An aluminum surface as in claim 17 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       19. An aluminum surface as in claim 17 or 18 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       20. An aluminum surface as in claim 2, 5 or 8 wherein the coating solution employed to coat the surface contains from about 0.5×10 -3  moles/liter to about 1.75×10 -3  moles/liter of zirconium and from about 0.3×10 -3  moles/liter to about 1.75×10 -3  moles/liter of polyhydroxy compound. 
     
     
       21. An aluminum surface as in claim 20 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       22. An aluminum surface as in claim 20 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       23. An aluminum surface as in claim 20 wherein nitric acid is present in the coating solution employed to coat the surface and the solution has a pH of about 3.0 to about 4.0. 
     
     
       24. An aluminum surface as in claim 11, 14 or 17 wherein the coating solution employed to coat the surface contains from about 0.5×10 -3  moles/liter to about 1.75×10 -3  moles/liter of zirconium and from about 0.3×10 -3  moles/liter to about 1.75×10 -3  moles/liter of polyhydroxy compound. 
     
     
       25. An aluminum surface as in claim 24 wherein the coating solution employed to coat the surface has a pH of about 3.0 to about 4.0. 
     
     
       26. An aluminum surface as in claim 24 wherein nitric acid is present in the coating solution employed to coat the surface. 
     
     
       27. An aluminum surface as in claim 24 wherein nitric acid is present in the coating solution employed to coat the surface and the solution has a pH of about 3.0 to about 4.0. 
     
     
       28. A continuous process for coating aluminum surfaces having a bright shiny appearance which comprises contacting such surfaces with an acidic aqueous coating solution having a pH within the range of about 3.0 to about 5.0 and consisting essentially of at least about 0.5×10 -3  moles/liter of zirconium or hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of a polyhydroxy compound having no more than 7 carbon atoms, and sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution and provide uncomplexed available fluoride, said solution being free of boron and phosphate, and replenishing said solution as necessary with aqueous replenishing concentrate so as to maintain such concentrations, said replenishing concentrate also being free of boron and phosphate, so as to continuously produce aluminum surfaces having uniformly colorless and clear coatings which are capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for 5 minutes, but which resist blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F., wherein said polyhydroxy compound is selected from the group consisting of gluconic acid, a salt of gluconic acid, sorbitol, mannitol, dextrose, ethylene glycol, glycerine and glucoheptonate. 
     
     
       29. A process as in claim 28 wherein zirconium is present in the coating solution employed to coat the surfaces and the temperature of the coating solution is at least about 110° F. to about 160° F. 
     
     
       30. An aluminum surface as in claim 29 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       31. A process as in claim 28, 29 or 30 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       32. A process as in claim 29 wherein the source of zirconium is ammonium fluozirconate. 
     
     
       33. A process as in claim 32 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       34. A process as in claim 32 or 33 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       35. A process as in claim 29 wherein the source of zirconium is fluozirconic acid. 
     
     
       36. A process as in claim 35 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       37. A process as in claim 35 or 36 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       38. A process as in claim 29 wherein the polyhydroxy compound is selected from the group consisting of gluconic acid and salts of gluconic acid. 
     
     
       39. A process as in claim 38 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       40. A process as in claim 38 or 39 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       41. A process as in claim 32 wherein the polyhydroxy compound is selected from the group consisting of gluconic acid and salts of gluconic acid. 
     
     
       42. A process as in claim 41 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       43. A process as in claim 41 or 42 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       44. A process as in claim 35 wherein the polyhydroxy compound is selected from the group consisting of gluconic acid and salts of gluconic acid. 
     
     
       45. A process as in claim 44 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       46. A process as in claim 44 or 45 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       47. A process as in claim 29, 32 or 35 wherein the coating solution employed to coat the surfaces has a temperature of from about 130° F. to about 150° F. and contains from about 0.5×10 -3  moles/liter to about 1.75×10 -3  moles/liter of zirconium and from about 0.3×10 -3  moles/liter to about 1.75×10 -3  moles/liter of polyhydroxy compound, and the replenishing concentrate consists essentially of from about 30.8×10 -3  moles/liter to about 250.5×10 -3  moles/liter of zirconium, from about 18.9×10 -3  moles/liter to about 148.0×10 -3  moles/liter of polyhydroxy compound, and a material which is a source of about 89.5×10 -3  moles/liter to about 695.0×10 -3  moles/liter of uncomplexed available fluoride. 
     
     
       48. A process as in claim 47 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       49. A process as in claim 47 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       50. A process as in claim 47 wherein nitric acid is present in the coating solution employed to coat the surfaces and the solution has a pH of about 3.0 to about 4.0. 
     
     
       51. A process as in claim 38, 41 or 44 wherein the coating solution employed to coat the surfaces has a temperature of from about 130° F. to about 150° F. and contains from about 0.5×10 -3  moles/liter to about 1.75×10 -3  moles/liter of zirconium and from about 0.3×10 -3  moles/liter to about 1.75×10 -3  moles/liter of polyhydroxy compound, and the replenishing concentrate consists essentially of from about 30.8×10 -3  moles/liter to about 250.5×10 -3  moles/liter of zirconium, from about 18.9×10 -3  moles/liter to about 148.0×10 -3  moles/liter of polyhydroxy compound, and a material which is a source of about 89.5×10 -3  moles/liter to about 695.0×10 -3  moles/liter of uncomplexed available fluoride. 
     
     
       52. A process as in claim 51 wherein the coating solution employed to coat the surfaces has a pH of about 3.0 to about 4.0. 
     
     
       53. A process as in claim 51 wherein nitric acid is present in the coating solution employed to coat the surfaces. 
     
     
       54. A process as in claim 51 wherein nitric acid is present in the coating solution employed to coat the surfaces and the solution has a pH of about 3.0 to about 4.0. 
     
     
       55. A acidic aqueous coating solution having a pH within the range of about 3.0 to about 5.0 and consisting essentially of at least about 0.5×10 -3  moles/liter of zirconium or hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of polyhydroxy compound having no more than 7 carbon atoms, and at least sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution, said solution being free of boron and phosphate and capable of forming a uniformly colorless and clear coating on an aluminum surface which is capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for about 5 minutes, but which resists blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F., wherein said polyhydroxy compound is selected from the group consisting of gluconic acid, a salt of gluconic acid, sorbitol, mannitol, dextrose, ethylene glycol, glycerine, and glucoheptonate. 
     
     
       56. An aqueous concentrate such that an aqueous coating solution containing about 0.5 to about 10 weight percent of the concentrate has a pH within the range of about 3.0 to about 5.0 and consists essentially of at least about 0.5×10 -3  moles/liter of zirconium or hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of a polyhydroxy compound having no more than 7 carbon atoms, and at least sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution, said coating solution being free of boron and phosphate and capable of forming a uniformly colorless and clear coating on a aluminum surface which is capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for 5 minutes, but which resists blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F., wherein said polyhydroxy compound is selected from the group consisting of gluconic acid, a salt of gluconic acid, sorbitol, mannitol, dextrose, ethylene glycol, glycerine, and glucoheptonate. 
     
     
       57. An aluminum surface having a uniformly colorless and clear coating which is free of boron and phosphate and is capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for 5 minutes, but which resists blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F., said coated aluminum surface having been obtained by contacting such aluminum surface with an acidic aqueous coating solution having a pH within the range of about 3.0 to about 5.0 and consisting essentially of at least about 0.5×10 -3  moles/liter of zirconium or hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of a water soluble polyhydroxy compound having no more than 7 carbon atoms, and at least sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution. 
     
     
       58. A continuous process for coating aluminum surfaces having a bright shiny appearance which comprises contacting such surfaces with an acidic aqueous coating solution having a pH within the range of about 3.0 to about 5.0 and consisting essentially of at least about 0.5×10 -3  moles/liter of zirconium or hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of a water soluble polyhydroxy compound having no more than 7 carbon atoms, and sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution and provide uncomplexed available fluoride, said solution being free of boron and phosphate, and replenishing said solution as necessary with aqueous replenishing concentrate so as to maintain such concentrations, said replenishing concentrate also being free of boron and phosphate, so as to continuously produce aluminum surfaces having uniformly colorless and clear coatings which are capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for 5 minutes, but which resist blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F. 
     
     
       59. An acidic aqueous coating solution having a pH within the range of about 3.0 to about 5.0 and consisting essentially of at least about 0.5×10 -3  moles/liter of zirconium and hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of a water soluble polyhydroxy compound having no more than 7 carbon atoms, and at least sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution, said solution being free of boron and phosphate and capable of forming a uniformly colorless and clear coating on an aluminum surface which is capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for about 5 minutes, but which resists blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F. 
     
     
       60. An aqueous concentrate such that an aqueous coating solution containing about 0.5 to about 10 weight percent of the concentrate has a pH within the range of about 3.0 to about 5.0 and consists essentially of at least about 0.5×10 -3  moles/liter of zirconium or hafnium or a mixture thereof, at least about 0.025×10 -3  moles/liter of a water soluble polyhydroxy compound having no more than 7 carbon atoms, and at least sufficient fluoride to combine with and form a soluble complex with all of the zirconium and hafnium present in the solution, said coating solution being free of boron and phosphate and capable of forming a uniformly colorless and clear coating on an aluminum surface which is capable of turning light golden brown to purple in color when heated at a temperature of 900° F. for 5 minutes, but which resists blackening for at least 5 minutes when heated in water having a temperature within the range of about 140° F. to about 170° F. 
     
     
       61. An aluminum surface as in claim 1 or 57 wherein said coating underlies an overlying water-borne coating. 
     
     
       62. A process as in claim 28 or 58 including applying to said coating an overlying water-borne coating. 
     
     
       63. An aluminum surface as in claim 61 wherein said water-borne coating is a polyester coating. 
     
     
       64. A process as in claim 62 wherein said water-borne coating is a polyester coating.